I just find a step by step guide on how to convert FLAC to iPhone 5S supported audio formats to get iPhone 5S to play .flac audio files. This also works for converting FLAC to iPhone 5C/5/4S/4/3GS/3G, iPad, iPod, iTunes, Quicktime and etc. It even applies to convert WMA, OGG, RA, AU, APE, AMR, CAF, QCP and etc to iPhone compatible audio format.

Another way is to convert FLAC to iPhone supported format. iDealshare VideoGo is just the professional FLAC to iPhone Converter which can fast and batch convert FLAC files to iPhone supported format. It can either convert FLAC to iPhone supported lossless audio format like Apple Lossless ALAC M4A, AIFF, WAV; or convert FLAC to iPhone supported lossy audio format like AAC or MP3.